This invention relates to a liquid crystal display device, and particularly to a twist nematic (TN) type liquid crystal display device.
A TN type liquid crystal display device will be described, referring to the accompanying drawings.
FIG. 1 is a cross-sectional view of one example of a TN type liquid crystal display device, in which a first substrate 1 and a second substrate 1' of transparent glass, etc. are juxtaposed at a predetermined clearance, for example, at 5-15 .mu.m, and their open peripheries are sealed with a sealing material 2, such as frit glass, and organic adhesive, etc. A nematic liquid crystal 3 is placed in a space formed by the substrates and the sealing material. The clearance can be provided by means of a spacer 4 of, for example, fiber glass, glass powder, etc. The sealing material 2 can serve also as a spacer without using any special spacer 4.
Electrodes 5 and 5' in a predetermined pattern are formed on the inner surfaces of the first and second substrates 1 1', respectively, and liquid crystal-aligning films 7, and 7' are further provided on the surfaces of the substrates to be in contact with a liquid crystal, and on the liquid crystal-aligning films are formed liquid crystal-aligning surfaces 6 and 6' capable of aligning the liquid crystal molecules in the vicinity of their surfaces into a desired direction. Such aligning surfaces 6 and 6' extend over the aligning films 7 and 7' of, for example, organic polymer on the substrates having the electrodes, respectively, and are formed by applying the so-called rubbing treatment to the surfaces, for example, by rubbing the surfaces with cloth in a predetermined direction.
In the TN type liquid crystal display device, a liquid crystal is filled into the space between glass substrates to form a liquid crystal layer, in which the liquid crystal molecules must be aligned substantially in parallel to the glass substrates and also can be twisted in a predetermined direction, usually in a direction at an angle of 90.degree., in the liquid crystal layer. This liquid crystal-aligning characteristic can be obtained by aligning films 7 and 7' having aligning surfaces 6 and 6', which are formed on the juxtaposed glass substrates. For the aligning films, usually organic polymers are used, and various organic polymers have been proposed, among which organic polymers containing an imide ring or a quinazoline ring have a good aligning property and can form an excellent aligning film whose aligning performance is not degraded even by a high temperature treatment.
On the other hand, the sealing material 2 is usually frit glass or an organic adhesive. To melt the frit glass as the sealing material 2, a temperature as high as about 400.degree. C. is required even for a low melting frit glass. When, for example, an aligning film comprised of a heat-resistant organic polymer containing at least one of the imide ring and the quinazoline ring is used as the aligning films 7 and 7' to this end, that is, when an organic polymer film having a heat resistance of higher than 400.degree. C. is used, deterioration will take place at a lower temperature at the parts in contact with the glass substrates 1 and 1', and consequently the attrition grooves made in one direction by rubbing will be damaged by the said heat treatment at 400.degree. C., or the organic polymer film itself will be dissipated thereby, and the aligning films 7 and 7' will lose the property of aligning the liquid crystal.
For example, it has been found by thermal balance analysis, ultraviolet analysis, etc. that a film of polyimidisoindroquinazolinedion (PIQ, a trademark of the product made by Hitachi Kasei Kogyo K.K., which will be hereinafter referred to as PIQ) as the organic polymer film has a heat resistance of about 450.degree. C. However, when a PIQ film having a thickness of about 1,000 .ANG. is provided each on soda glass substrates 1 and 1' without any undercoating film therebetween and when attrition grooves are provided on the surfaces of the PIQ films by cloth, etc. to obtain aligning films 7 and 7', as shown in FIG. 1, the property of aligning the liquid crystal molecules will be lost by heat treatment at about 350.degree. C. The heat deterioration will take place at parts in direct contact with the aligning films 7 and 7'. To prevent such deterioration, the undercoating films 8 and 8' are provided therebetween.
On the other hand, when an adhesive of organic polymer is used as the sealing material, the heat resistance will be no more problem owing to the low heat-treating temperature. However, the adhesive of organic polymer can be permeated by water, and the moisture endurance of aligning films will be a problem when used in a liquid crystal display device. When no undercoating films are used as in FIG. 1, no good adhesiveness between the aligning films 7 and 7' and the glass substrates 1 and 1' can be maintained in a moisture condition, and moisture permeates through the sealing material 2 from the outside into between the aligning films 7 and 7' and the glass substrates 1 and 1', thereby lowering the surface resistance of the glass substrates 1 and 1'. When the display parts are actuated by applying a voltage to the liquid crystal display device, the areas that should not be actuated around the display parts will be actuated. That is, an unwanted overdisplay phenomena (which will be hereinafter called "leaking") appears.
For example, as shown in FIG. 1, transparent electrodes 5 and 5' are formed on soda glass substrates 1 and 1' respectively, and PIQ films having a thickness of about 1,000 .ANG. are further formed thereon, respectively. Then, the PIQ films are made into aligning films 7 and 7', respectively, by providing attrition grooves thereon by rubbing. Then, the peripheral ends of the substrates are sealed with an epoxy adhesive, after spacers 4 are provided therebetween to provide a space for a liquid crystal. A liquid crystal 5, which comprises an azoxy liquid crystal as a host and an ester-based liquid crystal having a positive dielectric anisotropy, is filled into the space to provide a liquid crystal display device. The thus prepared device is subjected to a test under a moisture condition at 70.degree. C. and 95% relative humidity (RH). The so-called leaking appears after about 50 hours. This corresponds to a life of 6 months to one year under the normal environmental condition, which is a practical problem.
To solve the problem, undercoating films 8 and 8' are provided, as shown in FIG. 2. That is, undercoating films 8, or 8' of metal oxide is provided between the glass substrate 1 or 1' and the transparent electrode 5 or 5', and the aligning film 7 or 7' of organic polymer is formed on the subcoating film 8 or 8'. The heat resistance of the aligning film 7 or 7' and the adhesiveness between the aligning film 7 or 7' and the glass substrate 1 or 1' are improved by providing the undercoating film 8 or 8' therebetween. For the undercoating film, metal oxides, particularly silicon oxide, aluminum oxide, etc. are used (see Japanese Laid-open Patent Application No. 95264/79). Heretofor, liquid crystal display devices with high reliability free from any practical problem have been prepared in this manner. However, a step of providing an undercoating film is indispensable, and thus the process is complicated. The cost is inevitably increased.